This invention relates to printed circuits and methods of making them and, in particular, to solerable plating finishes therefor.
Many printed circuit boards with plated through holes, copper being plated onto the walls (barrels) of the holes, are made using a plate-on metal finish as an etch resist. This metal finish is plated on using a resist coating (photo-resist) which covers the areas of the board which will not form tracks or pads. The plated-on metal finish thus forms the final coating for the tracks, pads and barrels of the holes. After the metal has been plated and the resist coating is removed from the board, and the original copper cladding of the board, and any further copper plated over it, are etched away from the area which are not tracks or pads and are thus not protected by the plated-on metal finish etch resist.
The two main requirements for such a plated-on etch resist are that it should be resistant to as wide a variety of commonly used etchants for copper as possible and that it should be readily solderable. The use of metal which can form a thin, pore-free coating is desirable, as is low cost. Considerable saving in processing costs may be achievable if a plated-on metal etch resist, having sufficient corrosion resistance, low enough resistance, could be employed to also form a contact surface of an edge connector finger of the printed circuit board, or to form the underlay plating for a final coating of a thin layer of another metal, often gold, over an edge connector finger.
Gold has been used as an etch-resist/surface-finish plating, but its use can cause problems in solder joints due to gold embrittlement, and its cost makes use as an overall finish undersirable. Most metal-finished through-hole plated boards are, therefore, given a plated-on tin-lead alloy finish. This is obviously solderable and is resistant to many etchants, but it has some disadvantages. The coating has to be fairly thick, some 10 microns, to form a good resist and to avoid the formation of unsolderable copper-tin intermetallic compounds over the board surface. During etching, the etchant eats away the sides of the tracks and pads, leaving a small width of the tin-lead plating overhanging. This can break away to form slivers which can cause unwanted short circuits. Such overhang slivers are usually removed from new boards by melting the tin-lead in a reflowing process which allows it to flow over the sides of the tracks. In addition, tin-lead plating cannot be used as an underplate for gold edge connector fingers. Thus when gold plating is required at least the board edge must be masked, after tin-lead plating, apart from the edge connector fingers so that the tin-lead can be stripped therefrom, by dipping in a suitable medium, before the gold and any underplating of nickel, or other suitable metal, is applied. A further disadvantage of tin-lead as an etch resist and overall finish is that if a solder resist pattern is applied to a board over the tin-lead on the tracks, this solder resist is liable to crack as the board is wave or drag soldered and the tin-lead beneath the resist melts.
All of these disadvantages of tin-lead plating can be overcome by the recently proposed use of palladium-nickel, or other nickel alloys such as tin-nickel, as the etch resist. The nickel alloys can be plated to form very thin pore-free coatings. These coatings are so thin that they lack the mechanical strength to form slivers as the etchant undercuts the etch resist plating. They also form an excellent underplate for thin gold contact (edge connector) fingers, and they do not melt and flow beneath a solder resist coating.
However, nickel and its alloys are not easy to solder to, and may require the use of an activated flux to achieve good joints. Activated fluxes can leave corrosive residues and thus their use may be undesirable in many applications.